FIGURE 12.38 (a) Concentrations of CIO and (b) the HC1 deficit in February and March 1992 outside and inside the Arctic vortex (adapted from Webster et al., 1993b).

the chemistry (Lefevre et al., 1994; von der Gathen et al., 1995; Rex et al., 1998). Figure 12.40, for example, shows the depletion of ozone in the Arctic stratosphere in the winter of 1991—1992 as a function of the number of hours to which the air mass had been exposed to sunlight (von der Gathen et al., 1995). Depletion rates up to 10 ppb per hour in sunlight were observed (Rex et al., 1998). During this period, temperatures were sufficiently low for PSC formation during December and the first half of January. As seen in Fig. 12.41, mean ozone depletion rates of up to ~f.5% per day were observed during this period, a little less but similar in magnitude to the rates measured in the Antarctic polar vortex (von der Gathen et al., 1995).

Both denitrification and dehydration are very common in the Antarctic polar vortex, but they do not appear to be as common in the Arctic regions (e.g., see Ramaswamy, 1988; Fahey et al., 1990; Toon et al, 1990b; Arnold et al., 1992; Kawa et al., 1992a; Tuck et al., 1994; Santee et al., 1995; Van Allen et al., 1995; and Sugita et al., 1998). For example, Santee and co-workers (1995) have shown, using satellite-based data, that in the Antarctic polar vortex in 1992, gas-phase concentrations of HN03 and H20 were both very small in mid-August, at the CIO peak. As the temperature rose above that where evaporation of PSCs should have occurred, their concentrations remained small, suggesting that the atmosphere was both denitrified and dehydrated; circumstantial support for denitrification of the Antarctic stratosphere is also found in nitrate peaks

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